Weather forecasters are keeping a close eye on a potent La Niña event expected to peak in the winter. This sprawling ocean-atmosphere feature is expected to help produce a mild winter of 2020-2021 over much of the United States, together with strengthening drought across the Sun Belt well into the new year.

La Niña is also inspiring a growing set of researchers to look beyond the traditional limits of seasonal prediction. That’s because La Niña tends to hang around longer than El Niño, often returning or restrengthening for a second or even a third northern winter.

That asymmetry points to the potential for outlooks that may — at least in some years — extend well beyond the one-year limit of standard practice. Such forecasts could give more than a year’s notice of the likelihood of impacts such as a drought or enhanced hurricane activity.

The players and their impacts

El Niño and La Niña refer to a warming and cooling, respectively, of the uppermost part of the eastern tropical Pacific Ocean, together with a broad set of atmospheric extensions. This entire semi-cyclic phenomenon is known as the El Niño-Southern Oscillation, or ENSO.

ENSO events tend to emerge in the northern fall, peak by winter and fade in the spring. It’s often clear by late summer that either La Niña or El Niño will predominate during the next few months. About a third of all northern winters see neutral conditions, with neither El Niño or La Niña present.

Both El Niño and La Niña influence how much it rains or snows and how warm or cold it gets across broad areas — and even the amount of hurricane activity in the Atlantic and Pacific — so knowing what to expect from ENSO is of keen use to agricultural interests, energy traders and others.

Even outlooks that are only modestly better than chance in a single year can still provide long-term value to large-scale economic sectors.

“If you’re betting just on a single year, it’s not always going to be a safe bet, but over the long term we see these impacts pan out more often than not,” said Michelle L’Heureux, a climate scientist at the National Weather Service’s Climate Prediction Center. L’Heureux leads the creation of ENSO outlooks that are issued jointly around the middle of each month by the Weather Service and the International Research Institute for Climate and Society at Columbia University’s Earth Institute.

The unforeseen arrival of a record-setting El Niño event in 1982-1983, including destructive floods in California, kicked off efforts by the National Oceanic and Atmospheric Administration to provide advance warning of ENSO events. The tropical Pacific was soon slathered with dozens of buoys to monitor ocean and air temperature. The development of a long-range computer model known as the Climate Forecast System — which includes interactions among ocean, atmosphere, land and sea ice, going out up to nine months — allowed NOAA to launch its quantitative ENSO outlooks in the early 2010s.

La Niña’s tendency to persist

The tantalizing prospect of two-year La Niña predictions has emerged from studies over the past decade led by researchers Yuko Okumura (University of Texas at Austin), Clara Deser (National Center for Atmospheric Research) and Pedro DiNezio (University of Colorado at Boulder), among others.

It’s long been known that El Niño is most often a one-year affair, whereas about half of initial La Niña events recur or restrengthen for one or more additional years (see chart below).

During a warm El Niño event, the tropical Pacific discharges a massive amount of heat into the atmosphere, enough to raise global air temperature for a few months. The ocean then “recharges” during the next cool La Niña event, again sequestering heat beneath the ocean surface. Eventually, another El Niño event gets triggered, though this process is less well understood than the El Niño-to-La Niña sequence. This parade of ENSO events plays out atop the longer-term accumulation of oceanic heat from human-produced global warming.

One of the insights that’s emerged from intensive computer modeling and improved observations is that the strength of El Niño — as measured by the amount of warm water in the uppermost eastern tropical Pacific — plays a major role in how long the subsequent La Niña will last.

DiNezio pointed out that all three of the strongest El Niño years in modern times — 1982-1983, 1997-1998 and 2015-2016 — were followed by La Niña events that lasted two or more years.

In contrast, weaker El Niño episodes are more often followed by just a single year of La Niña.

Based on these insights, it’s already possible to build an experimental forecast going two years out from an El Niño event, especially a potent one. After strong El Niño conditions had taken hold in late 2015, DiNezio carried out climate model simulations going out to 2017, plus another set for an imaginary 1,800-year period spanning hundreds of ENSO events. Together, the results pointed to a 60 to 80 percent chance that La Niña would prevail in both 2016-2017 and 2017-2018.

DiNezio and colleagues published the findings in late 2017, just as a second year of La Niña was cresting as predicted.

“These results demonstrate the feasibility of predictions of the duration of La Niña,” the authors noted.

Recent work led by Xian Wu, a postdoctoral researcher at the National Center for Atmospheric Research, suggests that stronger first-year La Niña events are more likely to persist. Wu also found that conditions across the Atlantic, Pacific and Indian oceans affect the ultimate duration of both La Niña and El Niño events.

At NOAA, “we’re somewhat constrained by our model horizons, but there are folks investigating this,” L’Heureux said. A research workshop on multiyear climate prediction, now scheduled for the summer of 2021, will include several sessions on ENSO prediction.

Okumura noted in an email that some models do not yet capture the asymmetric durations of El Niño and La Niña. “This is one of the important remaining issues we need to address before promoting the implementation of long-range (up to two years) ENSO forecasts,” she added.

How will this La Niña evolve?

As for the current La Niña event, it’s registering solidly in the moderate range, based on cool sea surface temperatures in the benchmark Niño3.4 region of the eastern tropical Pacific.

The Niño3.4 values for October were the lowest in nearly a decade, and the eighth-lowest for any October since 1950, according to L’Heureux.

Models suggest the current event may soon be peaking, at least for 2020-2021. A new probability forecast of ENSO strength developed by NOAA shows a 24 percent chance that the event will peak in the “strong” range between now and January (with “strong” defined as Niño3.4 readings averaging at least 1.5 Celsius below the seasonal norm for the three-month period).

What’s odd is that this substantial La Niña isn’t arriving on the heels of a robust El Niño event. In fact, there wasn’t an El Niño at all in 2019-2020, according to L’Heureux. Sea surface temperatures did creep above the threshold for just long enough, but there wasn’t the coupling to the atmosphere required to qualify as El Niño, as explained by the CPC’s Emily Becker at the ENSO Blog.

“This La Niña is quite strong, but it was not led by a strong El Niño,” DiNezio said. “It came out of nowhere.”

Looking further out, the IRI/NOAA 9-month ENSO outlook issued Dec. 10 shows a good chance of La Niña segueing to neutral conditions by summer 2021, the end of the forecast period. Most of the major computer models agree with this prognosis, but a couple of them show La Niña persisting at least through next summer.

Researchers are seeing other mixed signals as well.

With the current La Niña so robust, “in my opinion, there is a good chance that it will turn into a multiyear event,” Okumura said. Meanwhile, DiNezio estimates the odds of a two-year event to be very low, based on the lack of a preceding El Niño event in 2019-2020.

If La Niña does last only a year, that would be good news for parts of the Southwest already facing the potential for grinding drought over the next few months. A 2017 study led by Okumura on multiyear La Niña events finds that U.S. drought tends to expand further into the nation’s midsection when La Niña extends to a second year.

Many residents of the hurricane-battered Gulf and Atlantic coasts will also be happy to wish La Niña good riddance. Its absence would lower the odds of another hyperactive Atlantic hurricane season. The 2020 season produced a record 30 named storms through its official end Nov. 30.

Longer-range La Niña forecasts could benefit global agriculture

The effects of both first- and second-year La Niña events shape the worldwide balance of agriculture in different ways, according to Weston Anderson, a postdoctoral researcher at the International Research Institute for Climate and Society. There are typically “winners” and “losers” for each major crop in varying parts of the world as El Niño and La Niña unfold.

Anderson recently surveyed colleagues around the world on the potential usefulness of two-year ENSO forecasts.

“The majority of the experts I polled (75 percent) agreed that forecasts made 9-24 months in advance would provide information useful for taking specific actions,” Anderson said in an email. Those could range from decisions on crop sequencing and rotation to long-range plans for marketing and distribution. Some of these actions would actually be more feasible in the nine-to-24 month interval than for shorter time periods, Anderson said — which underscores the potential value of such forecasts, once their skillfulness in an operational setting can be established.

So even if the current La Niña doesn’t follow the two-year plan, we might still get valuable guidance on multiyear La Niña events in the not-too-distant future.

Bob Henson is a meteorologist and journalist based in Boulder, Colo. His books include “The Thinking Person’s Guide to Climate Change” and “Weather on the Air: A History of Broadcast Meteorology.”